1. Field of the Invention
The present invention relates to a photo-curable composition, an ink composition primarily suitable for inkjet recording, and an inkjet recording method using the ink composition.
2. Related Art
As methods of recording images on a recording medium, such as paper, based on an image data signal, an electrophotographic method, sublimation and melting thermal transfer methods, and an inkjet recording method are known. The electrophotographic method requires processes of charging on a photoreceptor drum and exposure to form an electrostatic latent image, and therefore the system is complicated and manufacturing costs are high. With thermal transfer methods, although apparatuses are inexpensive, there are problems such that running costs are high and waste is generated since ink ribbons are used. On the other hand, the inkjet recording method is advantageous because recording apparatuses are inexpensive, and an image is directly formed on a recording medium by ejecting inks only on necessary image parts, so that ink can be efficiently used and running costs are inexpensive. Further, the inkjet recording method produces little noise, and is therefore excellent as an image recording method.
Ink compositions curable by irradiation with radiation such as UV rays, in particular, inkjet recording inks (radiation-curable inkjet recording inks), are required to have sufficiently high sensitivity and to be capable of producing high quality images. High sensitivity imparts various advantages such as high curability by radiation, a reduction in consumed electricity, an increase in the life of a radiation generator due to the reduction of a load on a radiation generator, and prevention of generation of a low molecular weight substance due to insufficient curing. When ink compositions, in particular, inkjet recording inks, are used as the image part of a planographic printing plate, the curing strength of the image part is heightened by high sensitivity and high printing durability can be obtained, and for these reasons they are preferable.
The acids of onium compounds, radical polymerization initiators, and cationic polymerization initiators are sparingly absorptive in wavelengths of 360 nm or greater, and thus curing properties conspicuously worsen when exposure is performed with light source having a wavelength longer than this. In particular, since conventional photo-polymerization initiators such as sulfonium salts and iodonium salts are sparingly absorptive of UV rays on the long wavelength side, curing properties worsen upon forming a thick curing film. Further, photo-curable compositions containing a large amount of white pigment, such as titanium oxide, and having absorption in UV region, have many problems of implementation.
To address these problems, sensitizers (sensitizing dyestuffs) have been examined. For example, as sensitizers of a diaryl iodonium salt photo-polymerization initiator, various dye derivatives have been reported to be effective, and thioxanthone derivatives are known in particular as inexpensive and general sensitizers (for example, refer to Japanese Patent Application Laid-Open (JP-A) No. 11-263804. However, thioxanthone derivatives alone are insufficient for photo-curing a photo-curable composition that uses additives such as a pigment or the like. Photo-curable compositions using anthracene compounds (refer to JP-A No. 11-199681) or 9,10-dialkoxyanthracene derivatives (for example, Japanese Patent No. 3437069) have also been examined as sensitizers. However, since these anthracene compounds and thioxanthone compounds are absorptive up to a long wavelength region, there is a problem that photo-curable compositions and cured products thereof tend to be yellowed.
It can be seen from the above that there is a desire for photo-curable compositions curable with high sensitivity by irradiation with actinic radiation, capable of forming a cured film excellent in adhesion to the surface of a solid such as a recording medium, and, when used with ink compositions, capable of restraining coloration of compositions.